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RE: Are Roof Top Units considered Dead or Live Loads?

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I always include about double the expected weight of roofing material to account for future re-roofing.  I think this is good practice since section 1510 of the 2000 IBC implies that a second roof may be added without a structural analysis.  I know many architects and roofers that take this as the gospel.

As to RTU's, I consider them as live loads.  This is because they can certainly be removed in the future.  In continuous framing systems, using them as a dead load can reduce the stresses on adjacent spans.  If the units are then removed, the stress levels on the adjacent spans can be too high.  I don't use LRFD, so I am not sure how I would handle the load factor issue.  I do make sure that the RTU loads do not get any reduction due to tributary area.  I usually include them in a separate load case from the true live loads.

Adam Vakiener, PE
Structural Engineer
Southern A&E, LLC
phone: 770-819-7777
 fax: 770-819-7770

From: "Garner, Robert" <rgarner(--nospam--at)>
To: "'seaint(--nospam--at)'" <seaint(--nospam--at)>
Subject: RE: Are Roof Top Units considered Dead or Live Loads?

Good point.

How many of us include re-roofing allowances in our roof design loads?

Bob Garner, S.E.

-----Original Message-----
From: James Cohen [mailto:jccpc(--nospam--at)]
Sent: Tuesday, January 04, 2005 7:17 AM
To: seaint(--nospam--at)
Subject: Re: Are Roof Top Units considered Dead or Live Loads?

I have always considered it a dead load. The value should be easily
determined and it is fixed. Roof structural support systems are often
designed for a specific layout of roof top units and any relocation or
change in load needs to be reanalyzed. The same holds true for the actual
roofing system. I have seen too many instances of single ply systems being
replaced with heavier torch down assemblies or addition of ballast without
reanalysis and consequent problems arising.

James Cohen, PE
James Cohen Consulting, PC

----- Original Message ----- From: "Daniel Boltz" <dboltz(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Tuesday, January 04, 2005 09:58 AM
Subject: Are Roof Top Units considered Dead or Live Loads?

I've had different answers from various Senior Engineers.  Some say that
since the RTU weight does not vary in magnitude and location, it is a DEAD
LOAD.  Others say that if the RTU is changed or moved in the future, it's
weight and location would be considered a LIVE LOAD.

Any help is appreciated.

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